Power of Accommodation

Unlike a camera lens that slides back and forth to focus, your eye lens focuses by changing its own shape. Tiny muscles called ciliary muscles squeeze or relax around it, and this ability to adjust focal length is called accommodation.

When you look at something far away, the ciliary muscles relax, the lens flattens and thins, and its focal length increases. When you look at something close, the muscles contract, the lens bulges and thickens, and its focal length decreases so the close object still lands sharply on the retina.

This flexibility isn't unlimited, though. The near point (about 25 cm for a young adult with normal vision) is the closest distance at which you can see an object clearly without straining. Try reading a page held closer than that and it blurs. The far point is the farthest distance you can see clearly — for a normal eye, that's infinity.

So a 'normal' eye is one that can see anything comfortably between 25 cm and infinity — that entire range is what accommodation buys you.